1. Field of the Invention
The present invention relates to a projection display apparatus, and more particularly to an optical system for use in a projection display apparatus.
2. Description of the Related Art
For the quick development of technology, the projection display apparatus are popularly used in various fields such as for videoconferences, product display, or family theaters. To fit the commercial trend, the projection display apparatus is more lightness, thinner, smaller and more compact to lower the cost and raise the market competition.
Referring to FIG. 1, an optical system 10 is known from U.S. Pat. No. 5,552,922. Between a digital micro-mirror device (DMD) and a projection lens 12 are two adjacent and separated first prism 13 and second prism 14. By a light source 15 providing a bundle of light beam 151, the light beam 151 impinges into one surface 131 of the first prism 13 and is directed to another surface 132 with an incidental angle larger than the critical total reflection angle. By the total reflection of surface 132, the light beam 151 travels through a third surface 133, and impinge to the digital micro-mirror device 11. Then, the micro-mirror array of the digital micro-mirror device 11 re-reflects the information images light beam 152 which provides by the digital micro-mirror device 11 into the third surface 133 of the first prism 13. With an incidental angle smaller than the critical total reflection angle, the light beam 152 impinges into the surface 132, passes through the surface 132, travels into the space adjacent to the first prism 13 and the second prism 14 and, then, impinges into a surface 141 of the second prism 14. Finally, the light beam 152 passes through the surface 142 of the second prism 14, and impinges into the projection lens 12 to form the projection images.
However, in the prior art, the light beam 151 emitting from the light source 15, before impinging into the projection lens 12, needs to travel through the impinging surfaces six times (by an example of a single optical path, as shown in a dotted line.) Each time of impinging, part of the light beam is reflected to cause loss. In addition, the space between the first prism 13 and the second prism 14 makes the relative transmission angles of the information image light beam 152 excessive to lower the transmissivity and cause loss too. Therefore, the total transmissivity of the prior art reaches only about 92%. Meanwhile, referring to FIG. 2, the micro mirror 111 of the digital micro device 11 pivots on diagonal and tilts to form the ON-state or the OFF-state with different angles. As the light source 15 impinges into the digital micro device 11 with the specific direction 153, to cooperate with the diagonal pivot property of the digital micro device 111, the apparatus of the prior art arranges the diagonal line 112 of the digital micro device 11 parallel to the upper end and the lower end of the third surface 133. As a result, the superficial area of the third surface 133 is increased. Not only raise the material cost, but also increase the back-focus of other optical devices. Cause the volume of the whole optical system larger and don't fit the commercial trend for lightness, thinness, smallness and compactness.